Electronic devices, such as flat panel displays and integrated circuits, commonly are fabricated by a series of process steps in which layers are deposited on a substrate and the deposited material is etched into desired patterns. The process steps commonly include plasma enhanced chemical vapor deposition (CVD) processes and plasma etch processes.
Plasma processes require supplying a process gas mixture to a vacuum chamber called a plasma chamber, and then applying electrical or electromagnetic power to excite the process gas to a plasma state. The plasma decomposes the gas mixture into ion species that perform the desired deposition or etch process.
In capacitively excited process chambers, the plasma is excited by RF power applied between an anode electrode and a cathode electrode. Generally the substrate is mounted on a pedestal or susceptor that functions as the cathode electrode, and the anode electrode is mounted a short distance from, and parallel to, the substrate.
Commonly the anode electrode also functions as a gas distribution plate for supplying the process gas mixture into the chamber. The anode electrode is perforated with hundreds or thousands of orifices through which the process gas mixture flows into the gap between the anode and cathode. The orifices are spaced across the surface of the gas distribution plate so as to maximize the spatial uniformity of the process gas mixture adjacent the substrate. Such a gas distribution plate, commonly called a “shower head”, is described in commonly assigned U.S. Pat. No. 4,854,263 issued Aug. 8, 1989 to Chang et al.
The RF excitation of the plasma may be accomplished by connecting one of the two electrodes (i.e. either the anode or the cathode) to the output of an RF power supply and connecting the other electrode to electrical ground. However, many commonly used plasma enhanced CVD and etch processes require RF excitation at two different frequencies simultaneously, with a high frequency RF power supply being connected to the anode electrode and a low frequency RF power supply being connected to the cathode electrode (or vice versa).
We found that connecting the cathode electrode (the susceptor) and the anode electrode (the gas distribution plate) to separate power supplies, so that neither electrode is electrically grounded, can degrade process performance because some RF power can be coupled between one of these electrodes and electrically grounded chamber components such as the electrically grounded chamber wall. This diverts RF power from the region between the susceptor and the gas distribution plate where the RF power is required to perform the desired plasma enhanced CVD or etch process. In the case of a CVD process for depositing a film such as silicon nitride or silicon oxide, we found that such diversion of RF power to a plasma behind or alongside the susceptor can produce a film having undesirable properties such as tensile film stress, low density, and excessive hydrogen content.